In a sectorized cellular system, just as in multi-beam communication satellites, groups of signals are to be transmitted in different directions or beams. In a three sector cellular base station conforming to AMPS or D-AMPS standards, for example, as many as twenty carriers per sector may need to be transmitted. The current method to transmit twenty carriers in each sector is to have effectively twenty single carrier transmitters and power amplifiers, the outputs of which are passively combined into a single antenna using either dissipative combiners (hybrid couplers), or multi-coupling filters. The former has the advantage of being able to combine carriers without regard to frequency spacing. Multi-coupling filters can only be built to combine 900 MHz signals with a 250 KHz guard space between them. Both methods are very lossy, compounded by the need to locate the hardware at the bottom of the mast to suffer a 3-6 dB cable loss. The resulting system is thus very expensive.
Consequently, there has been much discussion on the use of a common, multi-carrier transmitter power amplifier that could jointly amplify 20 carriers after combining at a lower level. The problem with this approach is intermodulation between the 20 carriers. Hitherto, specifications for intermodulation levels of -60 dB have been the aim. However, achieving -60 dB 3rd order intermodulation requires about 12 dB of input back-off, so that the power amplifier has to be designed for a saturated (peak) power of 16 times the mean power, leading to inefficiency. However, intermodulation from frequencies owned by one operator to other frequencies owned by the same operator does not need to be -60 dB.
FIG. 1 illustrates the difference between the traditional MCPA per sector and the Matrix power amplifier concept. As can be seen from FIG. 1, the Matrix power amplifier does not consist of separate amplifiers per sector, but rather jointly amplifies all 60 carriers. It is a 3-input, 3-output amplifier instead of the three, 1-input, 1-output amplifiers illustrated in FIG. 1a.
U.S. Pat. No. 3,917,998 to Welti entitled "Butler Matrix Transponder" describes an arrangement of N coupled power amplifiers for amplifying N signal paths. The N signal paths envisioned comprise the relaying of signals from at least one ground station to N locations on the earth using an orbiting satellite. The benefit of using coupled amplifiers over using a set of N non-coupled amplifiers is that the set of non-coupled amplifiers is limited to generating a power which does not exceed the peak power capability of a single amplifier in any signal path, whereas the technique which uses coupled amplifiers allows the generation of a power equal to the sum of the powers of all the amplifiers in any signal path, provided that all signal paths do not require higher than the mean power at the same time. As a result, signals that vary both above and below a mean power level are accommodated more efficiently due to a better statistical averaging of the power demanded by the N signal paths. The matrix power amplifier of the Welti patent is for use in frequency division multiple access (FDMA) applications, and provides the facility to vary the number of FDMA carrier frequencies used in each signal path and thus correspondingly the power needed in each signal path over a wide range.
A matrix power amplifier according to the Welti patent contains a butler matrix for combining a number N of input signals to be amplified to produce N different combinations of the input signals. In addition, a set of N power amplifiers are provided so that each amplifier amplifies one of the combinations to produce N amplified signals. The matrix power amplifiers also contain a butler matrix for combining the amplified signals to produce N outputs that are amplified versions of the original N input signals. The benefit as compared with simply amplifying the original N inputs in independent amplifiers is the ability, if instantaneously needed, to devote more than the power of a single amplifier to one of the N signal paths. In principle, the matrix power amplifier can deliver the sum of the power output of all amplifiers to a single output.
The characteristics of intermodulation generated by non-linearities in a matrix power amplifier are different than a single amplifier. It can be shown that third order intermodulation between signals input respectively to inputs I and J of the input butler matrix appears on the output numbers (2i-j).sub.N and (2j-i).sub.N of the output butler matrix. As a first step to reducing intermodulation in a matrix power amplifier, one embodiment of the present invention provides an excess number of amplifying paths so that outputs (2i-j) or (2j-1) or their corresponding inputs are not used for desired signal outputs, but are terminated in dummy loads. Thus, third order modulation between signals i and j will not be transmitted. This requires that the number of butler matrix input and output ports M be greater than the number of signals to be amplified N, wherein the remaining M-N signals are terminated in dummy loads.
In the matrix power amplifier disclosed in Welti, a butler matrix having a number of inputs and outputs equal to the number of signal inputs (N) forms N orthogonal linear combinations of the signals using passive combiners such as 180.degree. and 90.degree. hybrid or directional couplers. In the simplest example, a 2 input, 2 output butler matrix would take input signals S1 and S2 and form (S1+S2)/.sqroot.2 and (S1-S2)/.sqroot.2. Elementary power amplifiers then amplify the orthogonal combinations, and then the amplified outputs are combined in an inverse coupler matrix to recover the amplified version of the individual signals S1, S2, . . . SN.
Thus, there is a need for a method and apparatus for reducing intermodulation distortion in a multichannel transmit amplifier array.